NERVE PHYSIOLOGY Ass. Prof. Dr. Emre Hamurtekin EMU Faculty of Pharmacy

CELLULAR ELEMENTS CELLULAR ELEMENTS IN CNS GLIAL CELLS NEURONS

GLIAL CELLS Communication Cell division (+) 2 major groups: – Microglia – Macroglia Oligodendrocytes Schwann cells Astrocytes – fibrous – protoplasmic

NEURONS

AXONAL TRANSPORT Axoplasmic flow (Dynein & kinesin) Cell body maintains the functional and anatomic integrity of the axon. Orthograde transport: – From the cell body to toward the axon terminals – Fast (400 mm/day) and slow ( mm/day) axonal transport Retrograde transport: – From the nerve ending to the cell body – Used vesicles, NGF, some viruses – About 200 mm/day

AXONAL TRANSPORT

RESTING MEMBRANE POTENTIAL Na-K-ATPase 2K+ 3Na Na-channel K-channel K+ K Na+ K+ Na+ -70 mV Na+ NEURON

ACTION POTENTIAL Step 1: Resting membrane potential Step 2: Some of the voltage-gated Na-channels open and Na enters the cell (threshold potential) Step 3: Opening of more voltage-gated Na-channels and further depolarization (rapid upstroke) Step 4: Reaches to peak level Step 5: Direction of electrical gradient for Na is reversed + Na- channels rapidly enter a closed state “inactivated state” + voltage – gated K-channels open (start of repolarization) Step 6: Slow return of K-channels to the closed state (after- hyperpolarization) Step 7: Return to the resting membrane potential

ACTION POTENTIAL Decreasing the external Na concentration has little effect on RMP, but reduces the size of action potential. Hyperkalemia: neuron becomes more excitable Hypokalemia: neuron becomes hyperpolarized. Hypocalsemia: increases the excitability of the nerve Hypercalsemia: decreases the excitability

ACTION POTENTIAL Once threshold intensity is reached, a full action potential is produced. The action potential fails to occur if the stimulus is subthreshold in magnitude. Further increases in the intensity of the stimulus produce no other changes in the action potential. So, the action potential is all or none in character.

ACTION POTENTIAL Absolute refractory period: From the time the threshold potential is reached until repolarization is about one-third complete. Relative refractory period: From the end of absolute refractory period to the start of after–depolarization.

RESTING MEMBRANE POTENTIAL Na-K-ATPase 2K+ 3Na Na-channel K-channel K+ K Na+ K+ Na+ -70 mV Na+ NEURON

CONDUCTION of the ACTION POTENTIAL Unmyelinated axon: – Positive charges from the membrane ahead and behind the action potential flow into the area of negativity. – By drawing off (+) charges, this flow decreases the polarity of the membrane ahead of the action potential. – This initiates a local response. – When the threshold level is reached, a propagated response occurs that in turn electronically depolarizes the membrane in front of it.

CONDUCTION of the ACTION POTENTIAL Myelinated axon: – Myelin is an effective insulator. – Depolarization travels from one node of Ranvier to the next. – This jumping of depolarization from node to node is called “saltatory conduction” – Faster than unmyelinated axons.

ORTHODROMIC & ANTIDROMIC CONDUCTION Orthodromic: From synaptic junctions or receptors along axons to their termination. Antidromic: The opposite direction (towards the soma)

NERVE FIBER TYPES & FUNCTION FIBER TYPEFUNCTIONFIBER DIAMETER (µm) CONDUCTION VELOCITY (m/s) MYELINATION AαAα Proprioception, somatic motor Myelinated AβAβ Touch, pressure Myelinated AγAγ Motor to muscle spindles Myelinated AδAδ Pain, temperature Myelinated B Preganglionic, autonomic ˂33-15Myelinated C, Dorsal root Pain, temperature 0,4-1,20,5-2Unmyelinated D, Sympathetic Postganglionic sympathetic 0,3-1,30,7-2,3

NERVE FIBER TYPES & FUNCTION Susceptibility ToMost SusceptibleIntermediateLeast Susceptible HypoxiaBAC PressureABC Local anestheticsCBA